The present invention generally relates to light source devices, methods of producing the light source devices, and display apparatuses, and, more particularly, to a light source device provided with discharge tubes that discharge electricity to emit light in a thin atmosphere, a method of producing such a light source device, and a display apparatus utilizing such a light source device.
Conventionally, the backlight of a display apparatus such as a liquid display apparatus is formed with a light source device that includes one or a plurality of discharge tubes and a reflector. The discharge tubes are cold cathode tubes, in which mercury is sealed in Ar gas or Ne gas. A fluorescent material is applied to the walls of the discharge tubes. The mercury gas generates ultraviolet rays during electric discharge, and the ultraviolet rays impinging onto the fluorescent material cause visible rays.
Most of the backlights of conventional liquid crystal display apparatuses are provided with a light guide plate. For instance, a light guide plate is flanked by two light source devices in a conventional liquid crystal display apparatus. Each of the light source devices includes two discharge tubes and a reflector. The two discharge tubes each having a diameter of several millimeters are arranged in a narrow space having a width of less than 10 millimeters. In this arrangement, the surrounding temperature of the discharge tubes often rises to 70° C. or higher. The temperature rise in the vicinity of the electrodes of the discharge tubes is particularly large. When the supply current is increased to obtain a greater luminance, the temperature of the neighboring area of the electrodes rises to 120° C. or higher.
The amount of light emission of the discharge tubes tends to decreases at a high temperature. Regarding the ultraviolet ray generating rate of the mercury gas, the concentration of the mercury gas varies in proportion to the amount of electric current. Meanwhile, the mercury gas absorbs the ultraviolet rays, and the absorbing rate varies exponentially with the product of the mercury gas concentration and the transmission distance. In other words, the transmittance varies as the concentration and the transmission distance increase.
The ultraviolet rays are converted into visible rays by the fluorescent material applied to the walls of the discharge tubes. The incident rate of one ultraviolet-ray photon impinging upon the fluorescent material is equal to the product of the diameter of each discharge tube and the concentration of the mercury gas. To sum up the above facts, the amount of visible ray emission can be expressed as follows:I=k×(J×n)×exp(−b×n×d)  (1)
Where I represents the amount of visible ray emission, d represents the diameter of each discharge tube, n represents the concentration of the mercury gas (a function of the temperature of the discharge tubes), J represents the amount of electric current, and k and b represent proportional constants. According to the equation (1), I takes the maximum value with a predetermined mercury gas concentration value n. When the concentration of the mercury gas exceeds the predetermined concentration value n, the amount of visible ray emission decreases. Since the concentration of the mercury gas varies exponentially with the temperature of the mercury gas, the luminance decreases at a high temperature as the temperature of each discharge tube increases.
Also, the temperature of each discharge tube increases with the amount of electric current. At a certain environmental temperature, the amount of visible ray emission decreases, even when the amount of electric current increases. Such a decrease of visible ray emission causes a problem in maintaining the luminance level of the backlight.
In the light source device, the electrodes provided at both ends of the discharge tubes have the highest temperature when the discharge tubes are turned on. To cool both ends of the discharge tubes, heat conductive members (heat conductive rubber caps, for example) are provided at both ends of the discharge tubes, and are engaged with the reflector.
However, the contact between the heat conductive members (the heat conductive rubber caps) and the discharge tubes is often insufficient, as shown in FIG. 2. Because of the insufficient contact, sufficient cooling effects cannot be obtained, resulting in a temperature rise in the neighboring area of the electrodes up to 130 or 140° C.
The electrode terminals of the discharge tubes are normally soldered and fixed to a wire harness for power supply. The soldering position is normally located in the vicinity of the heat conductive members or within the heat conductive members. When the soldering position has a high temperature, alloy crystals having 2 phases of Sn—Pb develop in the soldering position. When a stress exits between the harness and the discharge tubes, cracks develop on the grain boundaries among the alloy crystal grains, resulting in a rupture (also referred to as a creep phenomenon). The temperature at which the creep phenomenon occurs is referred to as a creep temperature.
As a means for maintaining a constant level of luminance, the concentration of the mercury gas within the discharge tubes can be made uniform. Since the concentration of the mercury gas varies depending on the temperature of the discharge tubes, the discharge tubes should be partially cooled so as to stabilize the luminance.
More specifically, a heat conductive member having a heat release function is provided for the discharge tubes, so that the concentration of the mercury gas at the attachment position of the heat conductive member can be increased. With this heat conductive member, the temperature of the discharge tubes at the attachment position of the heat conductive member can be controlled to obtain the optimum concentration of the mercury gas. In this manner, the maximum amount of light emission can be constantly obtained from the discharge tubes.
With the above light source device in which the partial cooling is performed on the neighboring area of the electrodes, however, there are problems when the-neighboring area of the electrodes becomes too cold.
More specifically, in the light source device on which the local cooling is performed, the neighboring area of the electrodes that is cooled might have the lowest temperature in the discharge tubes. In such a case, the control mechanism for controlling the concentration of the mercury gas does not function at all. When the temperature of the neighboring area of the electrodes becomes equal to or lower than the inner temperature of the discharge tubes at the locally cooled location, the concentration of the mercury gas becomes highest in the neighboring area of the electrodes. With the high concentration of mercury in the vicinity of the electrodes, the mercury exhausts at a quicker rate, resulting in a reduction of life of the discharge tubes.
In another case, the heat conductive member having the heat releasing function may not be sufficient for local cooling. In such a case, the luminance becomes uneven, and the concentration of mercury becomes higher in the vicinity of the electrodes when the temperature of the neighboring area of the electrodes becomes lower than the temperature of the attachment position of the heat conductive member.